In known CT systems X-rays are generated by directing a beam of accelerated electrons at an anode plate typically of tungsten. A certain depth of penetration is associated with the X-ray generation in the anode, so that the emitted radiation passes varying amounts of anode material before leaving the plate.
As a consequence, the intensity and the spectrum of the generated radiation show a cone-angle dependent variation, leading to detector row dependent beam hardening and intensity, the so-called well known heel effect.
Especially for cone angles spanned by the upcoming 256 row detectors, this heel effect is significant and needs to be corrected, so as to avoid image artifacts. An X-ray tube source generally includes an anode side and a cathode side. The anode side is also known as the target, which is bombarded with electrons to generate X-ray beam radiation. X-rays from the X-ray tube are generated at a small depth inside the target (anode) of the X-ray tube. X-rays traveling toward the anode side of an object being scanned travel through more volume of the target than X-rays traveling toward a cathode side of the object. Therefore, X-rays traveling toward the anode side leave the target more attenuated than X-rays traveling toward the cathode side.
Mori et al. introduce a heel effect compensating filter, which is placed in front of an X-ray beam and has a varying thickness along a anode-cathode direction or so-called z-direction. The filter is made of aluminum and is designed to equalize the beam intensity along the said direction. However, the filter can not correct the spectral distortions mentioned above (Mori et al., Prototype heel effect compensation filter for cone-beam CT, Phys. Med. Biol., 50 (2005) N359-N370).
As illustrated in U.S. Pat. No. 6,968,042 a filter with an anode side and a cathode side between an X-ray source and an X-ray detector, has a higher attenuation coefficient on the cathode side than on the anode side. The attenuation coefficient is determined to at least partially compensate for the target angle heel effect. The filter material is aluminum, copper, titanium or beryllium. However, due to the chosen material, the filter can not correct the spectral distortions mentioned above.